An LED component essentially comprises an LED chip (a piece of semiconductor material with appropriate wire leads) and a package (typically a substantially transparent material, configured in a dome shape, the dome acting as a lens for the emitted light).
The package must be rigid enough, at least on its exterior, to make the LED component structurally sound. On its interior, however, the package must be soft or resilient enough in the vicinity of the LED chip, that mechanical stresses do not damage the LED chip or the wire leads. Throughout, the transparent material of the package must keep light attenuation as small as possible, and must not adversely affect the spectrum of light which is emitted from the LED component.
Conventionally, it has been necessary to compromise between these different objectives. Materials such as PMMA (polymethylmethacrylate: acrylic or "plexiglas"), glass, polycarbonate, optical nylon, transfer molded epoxy, and cast epoxy have been used for encapsulation.
However, these materials suffer from the drawback that their optical transmissive characteristics degrade over time. As the material degrades, light is absorbed ("attenuation") to an increasing degree. In particular, light with shorter wavelengths, from the UV through the yellow, is absorbed. The result is called "yellowing", because the eye perceives the light, tending more toward longer wavelengths, as yellowish.
Note, incidentally, that there is also degradation due to decreased light output from the LED die itself.
Different LED components are configured for particular wavelengths of light. For instance, Red (.gtoreq.515 nm) and Yellow or Amber (.about.580-595 nm) LED technology are well-developed. By contrast, green through blue LEDs (400-570 nm) present design problems which have been more difficult to overcome.
Yellowing of the light-transmissive encapsulant material is not an issue for longer-wavelength Red, Amber, or Yellow LEDs. However, shorter-wavelength LEDs are particularly subject to attenuation because of yellowing.
Therefore, there is a need for an encapsulant for LEDs in the near UV, blue, and green range which avoids the drawbacks associated with yellowing and attenuation of the encapsulant material.